02-python-essentials/02.05-indexing-and-slicing.ipynb

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   "cell_type": "markdown",
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   "source": [
    "# 索引和分片"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 索引"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "对于一个有序序列，可以通过索引的方法来访问对应位置的值。字符串便是一个有序序列的例子，**Python**使用 `[]` 来对有序序列进行索引。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'h'"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s = \"hello world\"\n",
    "s[0]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Python**中索引是从 `0` 开始的，所以索引 `0` 对应与序列的第 `1` 个元素。为了得到第 `5` 个元素，需要使用索引值 `4` 。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'o'"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[4]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "除了正向索引，**Python**还引入了负索引值的用法，即从后向前开始计数，例如，索引 `-2` 表示倒数第 `2` 个元素："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'l'"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[-2]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "单个索引大于等于字符串的长度时，会报错："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "IndexError",
     "evalue": "string index out of range",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mIndexError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-4-79ffc22473a3>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0ms\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;36m11\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mIndexError\u001b[0m: string index out of range"
     ]
    }
   ],
   "source": [
    "s[11]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 分片"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "分片用来从序列中提取出想要的子序列，其用法为：\n",
    "\n",
    "    var[lower:upper:step]\n",
    "\n",
    "其范围包括 `lower` ，但不包括 `upper` ，即 `[lower, upper)`， `step` 表示取值间隔大小，如果没有默认为`1`。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello world'"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'el'"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[1:3]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "分片中包含的元素的个数为 `3-1=2` 。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "也可以使用负索引来指定分片的范围："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'ello wor'"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[1:-2]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "包括索引 `1` 但是不包括索引 `-2` 。\n",
    "\n",
    "lower和upper可以省略，省略lower意味着从开头开始分片，省略upper意味着一直分片到结尾。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hel'"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[:3]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'rld'"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[-3:]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello world'"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[:]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "每隔两个取一个值："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hlowrd'"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[::2]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "当step的值为负时，省略lower意味着从结尾开始分片，省略upper意味着一直分片到开头。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'dlrow olleh'"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[::-1]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "当给定的upper超出字符串的长度（注意：因为不包含upper，所以可以等于）时，Python并不会报错，不过只会计算到结尾。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello world'"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s[:100]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 使用“0”作为索引开头的原因"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 使用`[low, up)`形式的原因"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "假设需要表示字符串 `hello` 中的内部子串 `el` ：\n",
    "\n",
    "|方式|`[low, up)`|`(low, up]`|`(lower, upper)`|`[lower, upper]`\n",
    "|--|--|--|--|--|\n",
    "|表示|`[1,3)`|`(0,2]`|`(0,3)`|`[1,2]`\n",
    "|序列长度|`up - low`|`up - low`|`up - low - 1`|`up - low + 1`\n",
    "\n",
    "对长度来说，前两种方式比较好，因为不需要烦人的加一减一。\n",
    "\n",
    "现在只考虑前两种方法，假设要表示字符串`hello`中的从头开始的子串`hel`：\n",
    "\n",
    "|方式|`[low, up)`|`(low, up]`\n",
    "|--|--|\n",
    "|表示|`[0,3)`|`(-1,2]`|\n",
    "|序列长度|`up - low`|`up - low`|\n",
    "\n",
    "第二种表示方法从`-1`开始，不是很好，所以选择使用第一种`[low, up)`的形式。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 使用0-base的形式"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "> Just too beautiful to ignore.  \n",
    "----Guido van Rossum"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "两种简单的情况：\n",
    "\n",
    "- 从头开始的n个元素；\n",
    "    - 使用0-base：`[0, n)`\n",
    "    - 使用1-base：`[1, n+1)`\n",
    "\n",
    "- 第`i+1`个元素到第`i+n`个元素。\n",
    "    - 使用0-base：`[i, n+i)`\n",
    "    - 使用1-base：`[i+1, n+i+1)`\n",
    "\n",
    "1-base有个`+1`部分，所以不推荐。\n",
    "\n",
    "综合这两种原因，**Python**使用0-base的方法来进行索引。"
   ]
  }
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